The field of the present invention is two-stroke internal combustion engines of the type employing cylinders oblong in cross-section.
Two-cycle internal combustion engines employing non-circular cylinders and pistons have been devised in an effort to increase the wall of the cylinder available for porting relative to the overall displacement. By employing a non circular cylinder for the combustion chamber arrangement, the ratio of cylinder surface area to volume increases over that of a circular configuration. As a result, a proportionally greater amount of area made available for the ports of the two-cycle engine can be realized. The term oblong has been employed in the present disclosure as a generic reference to such non-circular cross sections such as ellipses, ovals and the like.
In employing an oblong cylinder arrangement, certain disadvantages were recognized as well as the aforementioned advantage. FIG. 3 illustrates a two-cycle oblong cylinder configuration known prior to the present development. In this prior arrangement, the intake in exhaust porting was located along the long portions of the cylinder while the scavenging porting was located at the ends of the cylinder arrangement. This arrangement provided ample area for the intake and exhaust but caused the scavenging ports to be located close together and aimed toward one another. This effect is more pronounced than in a circular cylinder only in the sense that the volume served by the scavenging ports has been increased disproportionately to the radius of curvature at the scavenging porting. It has been determined that the effective scavenging port area is determined by the cord or straight-line distance between the sides of the port opening. This effective port area is seen to be significantly smaller than the arc area one might otherwise consider.
A further difficulty encountered in such oblong arrangements is that the distance between the scavenging porting and the exhaust porting is disproportionately small compared to the overall volume of the cylinder. As a result, short circuiting of scavenged air/fuel mixture to the exhaust porting is experienced while combusted gases in other areas of the combustion chamber are not effectively exhausted.
An additional problem encountered by the use of an oblong cylinder arrangement is the increase in flame path to remote areas of the combustion chamber. With a circular cylinder, the distance to the outer periphery of the combustion chamber is uniform. With an oblong arrangement, this distance can vary substantially.
Finally, the conventional sealing rings of an internal combustion engine experience substantial additional forces and moments due to the extended lengths of portions of the ring structure. This is particularly true in engines of two-stroke design having ports through the wall of the cylinder. Passage of the piston ring across the margins of such porting creates added resistance, interference and consequently stress to the piston ring.
In view of the foregoing disadvantages, the advantage of an increased ratio of cylinder area for porting to combustion chamber volume can be substantially lost.